Angiogenesis is the formation of new capillary blood vessels leading to neovascularization. It is a complex process which includes a series of sequential steps including endothelial cell-mediated degradation of vascular basement membrane and interstitial matrices, migration of endothelial cells, proliferation of endothelial cells, and formation of capillary loops by endothelial cells. Solid tumors are angiogenesis-dependent; i.e., as a small solid tumor reaches a critical diameter, for further growth it needs to elicit an angiogenic response in the surrounding normal tissue. The resultant neovascularization of the tumor is associated with more rapid growth, and local invasion. Further, an increase in angiogenesis is associated with an increased risk of metastasis. Accordingly, inhibiting tumor angiogenesis and associated tumor growth is an attractive approach to treating cancer for a variety of reasons (reviewed in 1–3). For example, antiangiogenic therapy can potentially overcome three major problems associated with other anticancer therapies, i.e., the problems of drug resistance (4, 5), poor delivery (6, 7) and tumor heterogeneity. Therefore, there is a need for effective methods of inhibiting angiogenesis and the tumor growth associated with it. One approach has been antibody-based targeting of tumor vasculature. By using specific antibodies to target vessels that supply tumors with blood, selective reduction of tumor tissue is possible, but not without undesirable complications.
Endoglin (EDG) has been targeted in antibody-based methods of reducing tumor vasculature, as EDG is a proliferation-associated antigen on endothelial and leukemia cells (8–11). Its expression is up-regulated in tumor-associated vascular endothelium (9–15). The EDG molecule is a homodimer glycoprotein antigen which was initially identified as a human leukemia-associated cell membrane antigen (16, 17). Its expression is restricted to immature B-lineage acute lymphoblastic leukemia cells, myelomonocytic leukemia cells, endothelial cells and a few minor normal cells (16–20). EDG binds transforming growth factor β (TGF-β), specifically to TGF-β1 and TGF-β3, but it does not bind to TGF-β2 (21).
Importantly, EDG is essential for angiogenesis (22). Certain anti-EDG monoclonal antibodies (mAbs) react with tumor-associated vascular endothelium more strongly than with vascular endothelium in normal tissues (9, 12–15). Immunoconjugates of selected anti-EDG mAbs that weakly cross-react with mouse endothelial cells are effective for suppressing angiogenesis and tumors in mice (10, 15, 23). These mAbs correspond to externally induced autoantibodies (24). Anti-EDG EDG mAbs have been conjugated with deglycosylated ricin A-chain to obtain immunotoxins (10, 15) and with 125I to obtain radioimmunoconjugates (23). Although the anti-EDG immunotoxins showed strong antiangiogenic anti-tumor efficacy at the dose of 24 to 45% of the 50% lethal dose (LD50), they also exhibited strong toxicity in mice. For example, LD50 of SN6f, SN6j and SN6k immunotoxins ranged between 14.8 and 17.8 μg/g body weight (10, 15). Therefore, the therapeutic windows of these immunotoxins are relatively narrow and are accordingly difficult to administer safely. Thus, there is a need for an alternative method of using anti-EDG antibodies to increase the effectiveness of anti-tumor agents in inhibiting angiogenesis and tumor growth.